In many cases, the cause of the digestive upset is minor; however, vomiting and diarrhea can be symptoms of a serious illness.

To better understand the causes of a cat or dog’s vomiting or diarrhea, it’s useful to have a general idea of your pet’s digestive system and how it works.

The Workings of the Digestive System

The digestive system has the very important job of keeping your pet hydrated by bringing water into the body and supplying your pet with nutrients from food.

Mouth

The digestion process begins when your pet chews food. Saliva mixes with the food, and an enzyme in the saliva begins to break down the starches in what your pet has eaten.

Saliva also lubricates the chewed food, making it easier to pass through the digestive system.

Throat

The chewed food then begins to travel to the stomach via the esophagus. A process called peristalsis causes the muscles of the esophagus to contract in waves that push the chewed food down the length of the esophagus and into the stomach.

Stomach

Once in the stomach, the chewed food has its proteins broken down by a secretion from the lining of the stomach called pepsin. Hydrochloric acid is also produced in the stomach, and the acid further breaks down the food for easier digestion.

The food remains in the stomach for one to two hours, and muscular valves on either end of the stomach keep the food from escaping while it is being digested. Muscles in the stomach keep the food moving around among the acid and pepsin to further the process, and then the food passes into the small intestine.

Small Intestine

A long, muscular tube that loops around within the abdomen is the small intestine, and it is made up of three separate sections. As the food passes through each of these sections — the duodenum, jejunum and ileum — the gallbladder releases bile and the pancreas releases digestive enzymes. Bile reduces lumps of fat into smaller pieces, and the digestive enzymes break down the fat even further.

The digestive enzymes also break down sugars, starches and proteins. All the nutrients released from the digestive process are absorbed into the body through the small intestine’s lining, and the leftover waste passes through to the large intestine.

Large Intestine

The large intestine, also known as the colon, is a large, muscular tube that connects the small intestine and the anus.

In a process of peristalsis, similar to that of passing food through the esophagus, the muscles of the large intestine contract in waves to push the waste through the colon, where the excess moisture is absorbed; the feces, in solid form, is expelled from the body when the rectal valve muscles relax.

Got all that?

Now let’s talk specifically about vomiting and diarrhea in dogs and cats.

Causes of Vomiting

Vomiting in pets is typically caused by an irritant that affects either the stomach or the duodenum, which is the first section of the small intestine.

In order to rid the body of the irritant, the process of peristalsis occurs in reverse, so that the irritant works its way back up the esophagus where it is vomited out. Irritation is most often caused by a cat or dog eating something that is not digestible, such as a bone, a stick or synthetic material, such as plastic.

As Dr. John Harper, DVM, notes, “Young cats and dogs often chew on yarn, string, sticks, rubber and plastic, and other objects that can’t be digested, causing irritation and vomiting.”

Some common causes of vomiting in pets:

Human Foods:Human foods, particularly those that are greasy, can also cause irritation. In simple cases, the pet throws up and health returns to normal within hours. Some irritants may cause vomiting to occur periodically over a day or two before ceasing.

Foreign Object: In some cases, however, a bone, stick or other foreign object may get stuck in the digestive tract, which causes an obstruction. As the body tries to expel the object, the obstruction blocks its path, which further irritates the digestive tract. This can result in persistent vomiting; severe cases can cause projectile vomiting.

Viral or Bacterial Infection: Other common causes of vomiting include infections as a result of a virus or, less commonly, bacteria. Like humans, cats and dogs can get viral or bacterial infections that require medication.

Parasites: Another common cause of vomiting, which affects young pets in particular, is parasites. Parasites, such as roundworms or hookworms, can cause vomiting as the body tries to expel the worms.

Serious Illness: Vomiting is also a symptom of some serious illnesses, such as cancer, liver disease or kidney disease. Cancer, particularly in older pets, can spread to the digestive system, which may cause irritation or obstructions in the digestive tract. A disease of the liver or kidneys causes toxins to build up in the body that causes irritation of the digestive system leading to vomiting.

In the following video, you’ll learn much more about reasons cats throw up:

What to Look For in Vomit:

An examination of the vomit can provide clues as to the cause.

Red blood in your pet’s vomit indicates bleeding of the digestive tract relatively close to the esophagus.

Darker blood that has the appearance of coffee grounds indicates blood farther down the digestive tract.

Vomit tinged with yellow bile may indicate an irritation in the first section of the small intestine; it can also indicate that the animal’s stomach is empty.

White substances, particularly those with a spaghetti-like appearance, indicate parasites.

Diarrhea in Dogs and Cats

Diarrhea in pets usually results from an irritant occurring at the end sections of the small intestine or in the large intestine. Because the digestive process hasn’t been completed, the moisture has not been removed from the feces, which is why it is expelled in a very liquid state.

The most common causes of diarrhea in dogs and cats are the same as vomiting:

Parasites

Foreign objects

Food irritants

Fat and skin from human scraps are often a cause of diarrhea in dogs, and shouldn’t be fed to pets, according to Dr. Carol Osborne, DVM. She states, “If you’re not going to eat it, neither should your dog. You’re not going to eat the fat and many of you aren’t going to eat the skin or the scraps.”

Fat is harder for cats and dogs to digest, which can cause irritation in the colon resulting in diarrhea.

An obstruction in the lower end of the small intestine or in the colon can also cause diarrhea; an obstruction in this area of the digestive tract often causes small amounts of blood to be expelled rather than feces. A puncture of the bowel caused by an obstructed object can cause the toxic contents of the intestines to spill into the abdominal cavity, causing a serious and sometimes fatal condition known as peritonitis.

In the following video, a veterinarian discusses the causes, symptoms and treatment of diarrhea in pets:

What to Look For in Feces

If your cat or dog has diarrhea, examining the feces may give clues as to the cause.

Look for expelled foreign objects such as fabric or plastic.

Parasites often appear as white, noodlish objects.

Feces that is soft but otherwise normal typically indicates a minor colon irritation.

Fresh, red blood and mucus are often an indication of an injury to the colon, such as from an obstruction.

Thick, black, sticky feces may indicate an injury in the duodenum.

Severely runny feces accompanied by a severe stench may indicate that the feces was expelled before undergoing the complete digestive process in the small intestine.

If your cat or dog’s vomiting or diarrhea lasts for several days, you should bring your pet to the veterinarian to have a thorough examination to determine the exact cause.